Dry rot is typically the result of one of two processes. The first is UV damage from the sun, and the second is simple exposure to air. The rubber compounds break down due to exposure. Tire companies add carbon black to help with UV exposure, and a wax to prevent air exposure. The wax is throughout the tire, and apparently redistributes itself slowly through normal usage - rolling, flexing, and heating and cooling cycles. Tires that are not used typically dry rot faster than tires that are used.
Dry rot can be seen as cracks, primarily in the sidewall. If they are greater than 1/32" (0.8mm) in width then they are considered too rotted to safely use, and may result in leaks or blowouts.
Manufacturers typically indicate that tires should not be used past 6-10 years, as the protective compounds have almost always worn out by then and dry rot is very likely.
Water based tire protectants can extend the life of the tires a little - avoid petroleum based protectants, as they can remove the tire's own protective chemicals at the surface.
If storing a tire for an extended period of time using a water based tire protectant and storing it away from heat and light in a neutral position (not bearing weight, inflated if on a hub) will provide the greatest life. As dry rot is a natural expected process, though, it will still occur and cannot be prevented, only delayed a little.
Check the DOT number, the last 3 or 4 digits (4 digits for newer tires) show the date of manufacture. 2210 would indicate the 22nd week of 2010. If it's less than 6 years old and doesn't show cracks in the sidewall, or the cracks are few and under 1/32nd" (0.8mm) wide, then the tire is probably safe to use. If it's older than 6 years or has wider cracks, it's unsafe to use.
If the tire is only 1-2 years old, you may be able to store it for another few years, but keep in mind that they'll continue to dry rot after you start using them again, so if you think you'll get 3 years of active use out of the tire, and it's already 2 years old, then you can really only expect to store it for a year before you start losing remaining life on the tire.
So the question about store or sell should be made on the useful life left on the tire. If they are new and you drive a lot, you can probably store them for 2-3 years and still get their full life from them. If they are 2-3 years old, have decent tread left, and you don't drive very much, you may find they dry rot before you use up the tread. Variation between manufacturers and even between tires within a manufacturer may mean some tires don't dry rot for 10 years, while others are unusable after 6 years. It doesn't appear that there's an easy way to determine the difference, though.